A range is the set of hands a player can realistically have in a spot.
Because you usually don’t know someone’s exact cards, you play against a range, not a single hand.
UTG opens preflop. Their range is usually tighter and contains more strong hands (like big pairs and strong aces) than a Button open range.
After UTG opens and then calls a 3-bet, you can narrow their range to hands that make sense for that action (often medium pairs and strong broadways, with fewer weak hands).